U.S. Pat. No. 6,274,857 (the 857 patent), which is incorporated herein in its entirety, discloses a method of induction heat treatment of an irregularly shaped workpiece, such as a crankshaft. The 857 patent discloses the use of a mating pair of coil segments, one in an active electrical circuit and one in a passive electrical circuit, to induction harden components of the workpiece, such as a crankshaft's pin or main. FIG. 1 is a diagrammatic illustration of one configuration for accomplishing induction hardening as taught in the 857 patent. In the figure, ac current Ia flows through first inductor segment 107 as illustrated by the direction of the arrows (instantaneous ac current). The lines diagrammatically illustrating first inductor segment 107 are connected to a suitable ac power supply, making the first inductor segment an active electrical circuit formed from a single turn inductor segment. Coil segments 107a and 107b are provided in first inductor segment 107. Current Ia creates a magnetic flux field around the active inductor coil segment. Coil segments 109a and 109b are provided in second inductor segment 109, which is a passive electrical circuit formed from a single turn inductor coil segment. Magnetic flux concentrator segments 103a and 103b form a magnetic flux concentrator that couples magnetic flux surrounding the active inductor segment to the passive inductor segment and induces ac current Ib in second inductor segment 109 as illustrated by the direction of the arrows (instantaneous ac current).
FIG. 2(a) and FIG. 2(b) respectively illustrate one example of a prior art active (first) inductor segment 107 and passive (second) inductor segment 109 that can be used to realize the diagrammatic circuits in FIG. 1. In FIG. 2(a) power termination regions 122a and 122b provide a means for connecting the active inductor segment to a suitable high frequency ac power supply. Dielectric 411 can be used to provide sufficient electrical insulation between the two regions. In FIG. 2(a) and FIG. 2(b) interior through openings 117a and 117b, respectively, split the first and second inductor segments, respectively into two coil segments. Each of these coil segments has a partial opening, such as openings 121a and 121b in coil segments 107a and 107b, respectively. Each coil segment around its opening can be designed with inner and outer coil lips separated by a quench orifice, such as inner and outer coil lips 123b and 123a, respectively, between quench orifice 131 in coil segment 107a. FIG. 2(c) illustrates the prior art active and passive inductor segments 107 and 109, respectively, properly positioned to inductively heat treat two components of a workpiece, each of which is positioned within the opening formed around a pair of coil segments, namely first pair of coil segments 107a and 109a (openings 121a and 122a), and second pair of coil segments 107b and 109b. Magnetic flux concentrators 103a and 103b are placed around the magnetic flux concentrator coupling regions 119a and 119b of the first and second inductor segments, respectively. Dielectric 410 separates the coil-facing surfaces 115a and 115b of the first and second inductor segments, respectively. Depending upon the workpiece component being inductively heat treated in a particular opening formed around a pair of coil segments, through openings 117a and 117b can also serve as the situs (residence) for a non-heat-treated workpiece component that joins the one or two workpiece components together.
With efficient magnetic coupling, the magnitude of the induced current Ib in the passive inductor segment will be approximately equal in magnitude to, and 180 electrical degrees out of phase with, the active current Ia in the active inductor segment. Approximately equal magnitudes for currents Ia and Ib does not insure equal current densities across the width of a coil segment. Equal cross sectional current densities in the active and passive circuits' facing coil segments is essential for uniform induction heating of the component of a workpiece placed within an opening formed by an opposing pair of coil segments. Non-uniformity of the electrically conductive material that an inductor segment is made of, or deviation from exact parallel plane relationship between the facing surfaces of a pair of inductor segments, can result in non-uniform current densities across the cross sectional width of the inductor segment. FIG. 3(a) and FIG. 3(b) are partial cross sectional views of opposing first and second inductor segments, at line A—A in FIG. 2(c). FIG. 3(a) illustrates a typical pair of opposing inductor segments 107 and 109 with ideal uniform cross sectional current densities (dotted region) for currents Ia and Ib. FIG. 3(b) illustrates a more realistic situation wherein the opposing coil-facing surfaces 115a and 115b are not parallel to each other, and both cross sectional current densities for currents Ia and Ib are non-uniform. In this example, coil-facing surface 115b of inductor segment 109 is not parallel with coil-facing surface 115a of opposing inductor segment 107. Consequently, due to the electromagnetic phenomenon known as the proximity effect, induced current Ib density is greater in the cross sectional region with a smaller air gap, which in turn will result in a current density re-distribution of active current Ia.
Uniformity of current densities in opposing active and passive coil segments can be impacted by the presence of electrically conductive masses on the complex-shaped workpiece that are located adjacent to the workpiece component being inductively heat treated in an opening between a pair of coil segments. FIG. 4 illustrates a workpiece component 207 situated between a pair of opposing coil segments 107a and 109a, formed from coil lips 107a′ and 107a″, and coil lips 109a′ and 109a″, respectively. Workpiece component 207, which will be inductively heat treated, is joined to bounding adjoining workpiece components 206 and 208, which will not be inductively heat treated. If the workpiece is a crankshaft, then component 207 may be a pin or main (with or without an oil passage in it), and the bounding adjoining workpiece components represent non-symmetrical counterweights on the crankshaft.
As disclosed in the 857 patent, two or more identical number of turns can be provided for both the active and passive inductor segments. However active and passive multi-turn inductor circuits require higher operating voltages than that for an equivalent single-turn arrangement. The higher operating voltages introduce the potential for arcing between adjacent circuit conductors with a small air gap that decreases reliability and maintainability.
Therefore, an object of the present invention is inductor segments that minimize non-uniform distribution of current density across the inductor to achieve uniform induction heat treatment of a workpiece component of a complex-shaped workpiece.